The present invention relates to extrusion coating compositions for metal substrates that, after application, demonstrate excellent adhesion, weather-ability, barrier properties, and flexibility; to a method of extrusion coating a metal substrate; and to a metal article, such as a metal can or container, or a material of construction, like aluminum siding, having at least one surface coated with an adherent layer of an extrusion coating composition. An extrusion coating composition comprises: (a) a polyester having a weight average molecular weight of about 10,000 to about 50,000, and optionally, (b) a modifying resin, for example, an epoxy or phenoxy resin having an epoxy equivalent weight of about 500 to about 15,000. The extrusion coating composition is applied to a metal substrate as a film having a thickness of about 1 to about 40 microns.
It is well known that an aqueous solution in contact with an untreated metal substrate can result in corrosion of the untreated metal substrate. Therefore, a metal article, such as a metal container for a water-based product, like a food or beverage, is rendered corrosion resistant in order to retard or eliminate interactions between the water-based product and the metal article. Generally, corrosion resistance is imparted to the metal article, or to a metal substrate in general, by passivating the metal substrate, or by coating the metal substrate with a corrosion-inhibiting coating.
Investigators continually have sought improved coating compositions that reduce or eliminate corrosion of a metal article and that do not adversely affect an aqueous product packaged in the metal article. For example, investigators have sought to improve the imperviousness of the coating in order to prevent corrosion-causing ions, oxygen molecules and water molecules from contacting and interacting with a metal substrate. Imperviousness can be improved by providing a thicker, more flexible and more adhesive coating, but often, improving one advantageous property is achieved at the expense of a second advantageous property.
In addition, practical considerations limit the thickness, adhesive properties and flexibility of a coating applied to a metal substrate. For example, thick coatings are expensive, require a longer cure time, can be esthetically unpleasing and can adversely affect the process of stamping and molding the coated metal substrate into a useful metal article. Similarly, the coating should be sufficiently flexible such that the continuity of the coating is not destroyed during stamping and molding of the metal substrate into the desired shape of the metal article.
Investigators also have sought coatings that possess chemical resistance in addition to corrosion inhibition. A useful coating for the interior of a metal container must be able to withstand the solvating properties of a product packaged in the metal container. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products, like beer, by imparting an off-taste to the product.
Conventionally, organic solvent-based coating compositions were used to provide cured coatings having excellent chemical resistance. Such solvent-based compositions include ingredients that are inherently water insoluble, and thereby effectively resist the solvating properties of water-based products packaged in the metal container. However, because of environmental and toxicological concerns, and in order to comply with increasingly strict governmental regulations, an increasing number of coating compositions are water based. The water-based coating compositions include ingredients that are water soluble or water dispersible, and, therefore, cured coatings resulting from water-based coating compositions often are more susceptible to the solvating properties of water.
In addition, water-based coating compositions do not completely overcome the environmental and toxicological problems associated with organic solvents because water-based compositions typically contain two or more pounds of organic solvent per gallon of coating composition. The organic solvent is a necessary ingredient to dissolve and disperse composition ingredients, and to improve the flow and viscosity of the composition. Therefore, in order to entirely avoid the environmental and toxicological problems associated with organic solvents, investigators have sought solid coating compositions that can be applied to a metal substrate. To date, investigators have had difficulty in providing a solid coating composition that matches a liquid coating composition with respect to film uniformity, film appearance, and film performance.
In prior attempts to find a useful solid coating composition, investigators have tested powder coatings, laminated film coatings, radiation cure coatings, and extrusion coatings. A great deal of research has been performed using free film laminates of polymers such as polyethylene terephthalate (PET), polypropylene (PP), and polyethylene (PE). In this method, a preformed polymer film, about 10 to about 25 microns thick, is applied to the metal substrate. The film laminate method is a rapid method of coating a metal substrate, but the method is expensive and the coated metal substrate does not possess all of the properties required, or desired, by can, can end, and closure manufacturers.
Solid powder coatings also have been used to coat a metal substrate with a coating composition. However, the application of a thin, uniform coating to a metal substrate, i.e., less than 40 microns, is difficult to impossible using the powder coating method. Often, if a thin coating is applied to a metal substrate using a powder coating method, the coating has imperfections which cause the film to fail. Such failures are impermissible in the food and beverage container industry, which further require thin coatings that can withstand shaping of a flat, coated metal substrate into a can, can end, or closure.
Solid coating compositions also have been extruded onto a metal substrate, for example, as disclosed in European Patent No. 0 067 060, PCT publication WO 94/01224, and Smith et al. U.S. Pat. No. 5,407,702. The extrusion coating of a solid composition onto a metal substrate is complicated by the fact that the solid composition must be heated sufficiently to melt the composition for flow through the extrusion apparatus. The heating step can cause a premature cure of the coating composition, especially a thermoset composition, which makes extrusion onto the metal substrate difficult due to crosslinking in the extruder and can adversely affect the performance of the composition coated on the metal substrate.
In order to overcome the problem of premature curing, investigators have attempted to extrude thermoplastic coating compositions onto a metal substrate. These investigators also encountered serious problems, such as composition components having either too high of a molecular weight for easy, economical extrusion, or too low of a molecular weight thereby providing an extruded film that is too soft for many practical applications, such as on the interior or exterior of a food or beverage container. Therefore, many patents and publications in the field are directed to extrusion apparatus and extrusion methods which permit the application of such solid coating compositions to a metal substrate.
Investigators, therefore, have sought a solid, extrudable coating composition for use on the exterior and interior of food and beverage containers that exhibits the advantageous properties of adhesion, flexibility, chemical resistance, and corrosion inhibition, and that is economical and does not adversely affect the taste or other esthetic properties of sensitive foods and beverages packaged in the container. Investigators especially have sought useful extrusion coating compositions in order to reduce the environmental and toxicological concerns associated with organic solvents. In particular, investigators have sought a solid, extrusion coating composition for food and beverage containers (1) that meets increasingly strict environmental regulations, (2) has corrosion inhibition properties at least equal to existing organic solvent-based coating compositions, and (3) is easily extruded onto a metal substrate as a thin, uniform film. Such an extrusion coating composition would satisfy a long felt need in the art.
A present extrusion coating composition comprises: (a) a polyester, or blend of polyesters, and optionally, (b) a modifying resin. A present extrusion coating composition is a thermoplastic composition and is extrudable onto a metal substrate. Therefore, a crosslinking step, such as an additional heating step after extrusion of the composition onto the metal substrate or use of a crosslinking agent, is not required. A present extrusion coating composition is free of organic solvents, yet an extruded film demonstrates excellent coating properties, such as adhesion, hardness, and flexibility.
A solid, extrusion coating composition of the present invention contains no organic solvents, and, therefore, overcomes the environmental and toxicological problems associated with liquid coating compositions. The present thermoplastic extrusion coating compositions also provide a sufficiently flexible extruded coating such that the coated metal substrate can be deformed without destroying film continuity. In contrast, thermosetting compositions often provide a rigid cured film thereby making it difficult to impossible to coat the metal substrate prior to deforming, i.e., shaping, the metal substrate into a metal article, like a metal closure, can, or can end. Coating a metal substrate prior to shaping the metal substrate is the present standard industrial practice.
As an added advantage, it is envisioned that a present extrusion coating composition can be used on can ends, can bodies, and closures, thereby obviating the use of different coating compositions by container manufacturers. Furthermore, a present extrusion coating composition exhibits sufficient clarity, hardness, and mar resistance after application for use as a coating on the exterior of a metal container. Accordingly, an extrusion coating composition of the present invention has a more universal range of applications, such as for the interior coating of a metal container for food or beverage products, or for the exterior coating of a metal container or a material of construction, like aluminum siding; overcomes the environmental and toxicological concerns associated with a liquid coating composition; and overcomes disadvantages presented by other methods of applying a solid coating composition to a metal substrate.
The present invention is directed to extrusion coating compositions that, after application to the metal substrate, effectively inhibit corrosion of the metal substrate, do not adversely affect products packaged in a container having an interior surface coated with the composition, and exhibit excellent flexibility, barrier properties, weathering, chemical resistance, and adhesion. An extrusion coating composition of the present invention can be used on closures, can ends, and can bodies, and on container interiors and exteriors, as well as materials of construction, like aluminum siding and gutters. An extrusion coating composition effectively inhibits corrosion of ferrous and nonferrous metal substrates when the composition is extruded to a surface of the metal substrate.
A present extrusion coating composition comprises: (a) a thermoplastic polyester, or a blend of polyesters, having a weight average molecular weight (Mw) of about 10,000 to about 50,000, and optionally, (b) a modifying resin, such as an epoxy or phenoxy resin having an epoxy equivalent weight (EEW) of about 500 to about 15,000. The composition is free of organic solvents.
In particular, the present extrusion coating composition comprises: (a) about 50% to about 100%, by total weight of the composition, of a polyester having an Mw of about 10,000 to about 50,000, and preferably about 15,000 to about 40,000, or a mixture of such polyesters, and optionally, (b) 0% to about 25%, by total weight of the composition, of a modifying resin, for example, an epoxy or phenoxy resin having an EEW of about 500 to about 15,000, and preferably about 1000 to about 10,000, or an acrylic resin having an Mw of about 15,000 to about 100,000, or a polyolefin having an Mw of about 15,000 to about 1,000,000, or a mixture thereof. A present extrusion coating composition optionally can include: (c) 0% to about 50%, by total weight of the composition, of an inorganic filler, and (d) 0% to about 4%, by total weight of the composition, of a flow control agent.
In particular, a polyester included in extrusion coating composition is a thermoplastic polyester prepared from an acid, preferably terephthalic acid, isophthalic acid, or a mixture thereof, and an aliphatic diol. The polyester most preferably is a co-polyester containing terephthalic acid and isophthalic acid. The polyester has an acid value of 0 to about 150 mg (milligram) KOH (potassium hydroxide)/g (grams) and a hydroxyl value of 0 to about 150 mg KOH/g, a softening point of 140xc2x0 C. or greater, and a glass transition temperature (Tg) of about xe2x88x9230xc2x0 C. to about 120xc2x0 C. In addition, the polyester has a melt viscosity of about 10 to about 100 Pa.s (Pascal seconds) at 200xc2x0 C. or about 25 to about 200 Pa.s at 240xc2x0 C., and a melt flow index (MFI) of about 20 to about 800 g/10 min (minutes) at 200xc2x0 C. Blends, or mixtures, of polyesters also are useful in the composition and method of the present invention.
Components (a), and (b) and (c) and (d), if present, and other optional components are heated and intimately admixed to provide a homogenous extrusion coating composition. After cooling, the extrusion coating composition is comminuted into pellets having a particle diameter of about 1 to about 10 mm (millimeters), and preferably about 4 to about 8 mm.
As used here and hereinafter, the term xe2x80x9cextrusion coating compositionxe2x80x9d is defined as a solid coating composition including a polyester, an optional modifying resin, an optional filler, an optional flow control agent, and any other optional ingredients. The term xe2x80x9cextruded coating compositionxe2x80x9d is defined as an adherent polymeric coating resulting from extruding an extrusion coating composition onto a metal substrate.
Therefore, one important aspect of the present invention is to provide an extrusion coating composition that effectively inhibits the corrosion of ferrous and nonferrous metal substrates. An extrusion coating composition, after extrusion onto a metal substrate, provides an adherent barrier layer of an extruded coating composition that effectively inhibits corrosion, exhibits excellent flexibility and adhesion on the metal substrate, and does not adversely affect a product, like a food or beverage, that contacts the extruded coating composition. Because of these advantageous properties, an extruded coating composition can be used to coat the interior of food and beverage containers and overcome the disadvantages associated with conventional liquid compositions and with solid compositions applied by methods such as powder coating and lamination. An extruded coating composition comprises the polyester, and, if present, the modifying resin, the filler, and the flow control agent, essentially in the amounts these ingredients are present in the extrusion coating composition.
In accordance with another important aspect of the present invention, an extruded coating composition demonstrates excellent flexibility and adhesion to a metal substrate. The excellent adhesion of an extruded coating composition to a metal substrate improves the barrier and corrosion-inhibiting properties of the coating composition. The excellent flexibility of an extruded coating composition facilitates processing of the coated metal substrate into a coated metal article, like in molding or stamping process steps, such that the cured coating composition remains in continuous and intimate contact with the metal substrate. An extruded coating composition exhibits excellent chemical resistance and does not adversely affect a food or beverage packaged in a container having an interior surface coated with the cured coating composition. An extruded coating composition is sufficiently hard to resist scratching.
In accordance with another important aspect of the present invention, an extrusion coating composition of the present invention can be extruded onto a metal substrate to provide a uniform film of extruded coating composition having a film thickness of about 1 to about 40 microns, and preferably 2 to about 30 microns. Uniform films of such a small thickness have not been attainable using powder coating composition and methods. In addition, a present extrusion coating composition can be used both on the interior and exterior of can bodies and can ends, thereby obviating the need for a container manufacturer to use multiple coating compositions.
These and other aspects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments.